In many types of fuel burner applications, it is desirable to provide a purge time prior to initiating the flow of fuel and ignition thereof at the burner. During the purge period a blower is energized to dissipate any fuel vapor that may occupy the region adjacent to the burner because of some inadvertence such as a leaky fuel valve. The dissipation of residual fuel vapor eliminates the possibility of an explosion during an attempt to ignite newly released fuel.
Various types of systems for establishing purge periods have been both proposed and developed. However, none of these prior systems have proven fully satisfactory. For example, those employing mechanical timers are both bulky and expensive while electrical units generally fail to offer the reliability desired for safety equipment of this type. One problem associated with many prior electrical purge timers is a tendency to trigger a try for ignition prior to the expiration of a predetermined purge period. Such a malfunction can be caused, for example, by a failure of individual components in the electrical timing circuits employed or by the inadvertent triggering of electronic switches such as SCR's by extraneously introduced signals. Another disadvantage of many prior purge timers is their failure to reset instantly upon completion of a purge timing period. The failure of a purge timer to reset instantly is particularly troublesome in burner applications employing dual fuels. For example, when switching from an oil to a gas supply, flame at the burner is usually extinguished so that a new gas flame must be ignited. However, the rotational inertia of the fuel oil pumps typically employed in such systems will continue to supply oil for a few seconds resulting in the accumulation of oil vapor in the vicinity of the burner. To insure against detonation of this vapor when a try for gas ignition is made, a full purge period is required but will not be available if the timer was not reset upon completion of the purge timing period preceding the previous ignition attempt at the burner.
The object of this invention, therefore, is to provide an improved burner control system with an electrical purge timer that will reliably produce a predetermined purge period before each ignition attempt regardless of the condition of its individual components or the environment in which it is used.